1. Field of the Invention
The present invention relates to a non-directional frequency generator, and more particularly to a non-directional spark prevention apparatus for preventing sparks generated from a non-directional frequency generator.
2. Description of the Related Art
Generally, electronic appliances such as microwave ovens, etc., are designed to be driven solely by an alternating current (hereinafter called AC) power source, and accordingly have a shortcoming in that the electronic appliances can not be used in the places such as the outdoors, in vehicles such as ships, airplanes, etc., where the AC power source is not available. In order to solve such a problem, a non-directional frequency generator (hereinafter called NDFG) has been used to convert direct current (hereinafter called DC) into AC in the places where the AC power source is not available.
The NDFG usually uses relays or semiconductor elements for its converting operation into AC. The conventional semiconductor type NDFG circuit, however, has some problems of increasing manufacturing cost due to the expensive semiconductor elements, output loss of the semiconductor elements due to the switching operation, and excessive heat generation due to the output loss, etc.
In order to solve the above problems, the same applicant disclosed a NDFG utilizing a rotational AC converter to convert DC into AC in the Korean Patent Application Nos. 98-18589 (filed May 22, 1998) and 98-21117 (filed Jun. 8, 1998), which have not been opened to the public yet.
Hereinafter, the above NDFG will be briefly described as a related art with reference to the accompanying drawings.
FIG. 1 is a schematic view for showing the NDFG of a microwave oven driven by the DC power source according to the related art of the present invention. FIG. 2 is a view for showing the waveforms of the AC power source generated by the rotation of the NDFG, in which (a), (b), and (c) refer to the output waveforms of a first relay RY.sub.1, a second relay RY.sub.2, and a non-directional frequency generator.
Referring to FIG. 1, the NDFG 100 includes a motor 110 driven by the DC power source for generating rotational force, a commutator 130 rotated by the motor 110, and a plurality of brushes such as first, second, third, and fourth brushes 121-124 as shown in FIG. 1, which are in contact with the outer circumference of the commutator 130. The commutator 130 includes a conductive part which is divided into at least two parts 132a and 132b as shown in FIG. 1, but into an even number of parts. The commutator 130 includes an insulating part 133 of a predetermined width formed between the conductive parts 132a and 132b. The conductive parts 132a and 132b are in simultaneous contact with at least two neighboring brushes 121-124. The DC is applied to input sides of the first to fourth brushes 121-124, while the output sides of the first to fourth brushes 121-124 are connected with a high voltage transformer (hereinafter called HVT). The first and second relays RY.sub.1 and RY.sub.2 switch on/off the operation of the NDFG 100.
The operation of the NDFG 100 is as follows: The first and second relays RY.sub.1 and RY.sub.2 are in the on-state, and the commutator 130 is rotated by the DC. Accordingly, the brushes 121-124 in contact with the commutator 130 come in contact with the conductive part 132a, the insulating part 133, the conductive part 132b, and the insulating part 133 which are formed on the outer circumference of the commutator 130, sequentially.
More specifically, as the first brush 121 on the upper side of the commutator 130 comes in contact with the conductive part 132a, the electric current from the positive (+) terminal of the DC power source is inputted into the first brush 121, and flows through the conductive part 132a of the commutator 130 and the fourth brush 124, and to the upper portion of the primary coil 202 of the HVT downwardly to the lower portion of the primary coil 202 of the HVT. Then, the electric current is inputted into the second brush 122, and circulates through the conductive part 132b, the third brush 123, and to the negative (-) terminal of the DC power source.
Next, as the commutator 130 is further rotated and as the first brush 121 accordingly comes in contact with the insulating part 133, the electric current can not flow through the commutator 130.
Then the commutator 130 is further rotated to 90.degree., the electric current from the positive (+) terminal of the DC power source is inputted into the first brush 121, flows through the conductive part 132b of the commutator 130 and the second brush 122, reverses its direction, and flows from the lower portion of the primary coil 202 of the HVT to the upper portion of the primary coil 202 of the HVT. Then, the electric current is inputted into the fourth brush 124, flows through the conductive part 132a, and the third brush 123, and then circulates to the negative (-) terminal of the DC power source.
By the constant rotation of the commutator 130 of the NDFG, AC is generated at the primary coil 202 of the HVT in a manner as described above, then the AC is transmitted to a secondary coil of the HVT through the primary coil 202 thereof. Then, the HVT converts the normal voltage into a high voltage, and the magnetron MGT is driven by the high voltage stepped-up by the HVT.
While the AC power is generated as above, there are two periods that alternate with each other, i.e., a brush-on period in which the conductive part 132a or 132b of the commutator 130 comes in contact with the brushes 121-124 so that the electric current flows through the commutator 130, and a brush-off period in which the insulating part 133 of the commutator 130 comes in contact with any of the brushes 121-124 so that the electric current can not flow through the commutator 130. Meanwhile, during the brush-off period, the energy stored in the secondary coil of the HVT and a capacitor during the brush-on period is induced to the primary coil of the HVT. Accordingly, the voltage is induced during the brush-off period, generating backward current. Referring to FIGS. 2A through 2C, voltage waveforms VW and current waveforms CW induced from the secondary coil to the primary coil of the HVT during the brush-off period are shown. In FIGS. 2A and 2B illustrate direct current waveforms inputted while the first and second relays of the NDFG 100 are switched on, while FIG. 2C illustrates alternating current waveforms detected at the output side of the NDFG 100.
In addition to the voltage and current waveforms shown in FIGS. 2A through 2C, spark waveforms are also shown which are steeply falling at the beginning of the brush-off period. The spark waveforms suddenly fall when the brush-on period is changed over to the brush-off period during the operation of the circuit, which means the excessive spark is produced between the commutator 130 and the brushes 121-124 at the beginning of the brush-off period, i.e., when the brush-on period is changed over to the brush-off period.
Such a generation of sparks destabilizes the operation of the NDFG, and shortens the life time of the NDFG.